Exhaust gas discharged from internal combustion engines such as vehicle engines contains hazardous substances such as hydrocarbon (HC), carbon monoxide (CO), and nitric oxide (NOx). Since these substances cause air pollution, purification of exhaust gas is necessary.
Exhaust gas can be purified using exhaust gas purifying catalysts. In general, exhaust gas purifying catalysts are composed of a substrate and a catalyst layer disposed on the substrate. Such catalyst layer comprises a carrier and a catalyst metal supported on the carrier. Noble metals such as platinum, palladium, and rhodium are commonly used as catalyst metals.
For example, Patent Document 1 discloses an exhaust gas purifying catalyst for NOx purification. The catalyst comprises: a substrate; and a lower catalyst layer and an upper catalyst layer disposed on the substrate, wherein the lower catalyst layer comprises rhodium, and the upper catalyst layer comprises rhodium and at least one of platinum and palladium.
In addition, Patent Document 2 also discloses an exhaust gas purifying catalyst for NOx purification. The catalyst comprises: a substrate; and a lower catalyst layer and an upper catalyst layer disposed on the substrate, wherein the lower catalyst layer comprises rhodium, and the upper catalyst layer comprises platinum and palladium.